1. Field of the Invention
The present invention relates to a planar magnetic device for use in various high-frequency components, such as a choke coil and a transformer which are to be incorporated into a switching power supply.
2. Description of the Related Art
As is demanded in the so-called multimedia age which has come recently, various portable electronic apparatuses are made smaller, thinner, lighter and more efficient. This owes much to the increased integration density of electronic circuits, which has been made possible by advanced LSI technology, advancements in component-mounting technology, and the development of high-energy battery cells (e.g., lithium cell and nickel-hydrogen cells).
The power-supply section of such an electronic apparatus has a switching type power supply which is a stable one. It is considered difficult to reduce the size and weight of the switching type power supply, without impairing the high power-converting efficiency of the power supply. The size, weight and manufacturing cost of the switching type power supply remains the same, while the those of the other components of the electronic apparatus are successfully reduced. Inevitably the switching type power supply becomes increasingly responsible for the size, weight and cost of the apparatus.
To reduce the size and weight of the switching type power supply, the switching frequency of the power supply may be increased so that the power supply may incorporate a small power-supply component, such as a small inductor, a small transformer or a small capacitor. Here arises a problem. The higher the switching frequency, the greater the energy loss in the small power-supply component, and lower the power-converting efficiency of the switching type power supply. To enable the power supply to convert high-frequency power efficiently, it is absolutely required that the small power-supply component should have but a small energy loss. Further, magnetic components, such as an inductor and a transformer, can hardly be made thinner. It therefore remains difficult to provide a switching type power supply which is sufficiently thin.
To provide a switching type power supply which is very small and thin, it has been proposed that a planar inductor or transformer be used which comprises a planar coil and a soft-magnetic film. FIG. 1A shows a conventional planar inductor. The planar inductor has a planar coil 1 which is generally square as shown in FIG. 1B. As shown in FIG. 1A, the coil 1 is interposed between two insulating layers 2, which are sandwiched between two soft-magnetic layers 3.
The planar inductor has the frequency characteristic illustrated in FIG. 2. As the higher the frequency f increases, the equivalent series resistance R rapidly increases, while the inductance L remains almost unchanged. The quality factor Q remains less than 10. Any inductance element whose quality factor Q is more than 10 is generally considered a good one. The higher the quality factor, the better. It is therefore demanded that the quality factor Q of planar inductors be increased. The high-frequency loss in each soft-magnetic layer 3 and the high-frequency loss in the planar coil 1 are regarded as preventing an increase in the quality factor Q of the planar inductor. (High-frequency loss of soft-magnetic layer is an eddy-current loss or a hysteresis loss.)
A new type of a planar inductor has been invented, which is shown in FIG. 3. This inductor comprises two insulating films (not shown), a planar coil 4 interposed between the insulating films, and two soft-magnetic layers 5 provided on the insulating films, respectively. The planar coil 4 is oblate as a whole. The soft-magnetic layers 5 are made of uniaxial anisotropic material, have a hard axis of magnetization and are magnetized in rotation magnetization mode. The eddy-current loss made in the layers 5 is therefore small. As a result, a decrease of the high-frequency loss in the layers 5 can be well expected.
The planar inductor shown in FIG. 3 has the frequency characteristics illustrated in FIG. 4. As FIG. 4 shows, the quality factor Q of the planar inductor is less than 10, at the most.
The inventors hereof analyzed the high-frequency loss in planar inductors, each comprising two soft-magnetic layers, two insulating layers sandwiched between the soft-magnetic layers and a spiral planar coil interposed between the insulating layers. The results of the analysis were as follows:
An inductor shown in FIG. 5A, comprising two soft-magnetic layers 8, two insulating layers 7 interposed between the layers 8 and a spiral planar coil 6 interposed between the insulating layers 7, had an internal magnetic flux. The flux consisted of an in-plane component Bi and a vertical component Bg, with respect to the soft-magnetic layers 8. These components Bi and Bg were distributed as illustrated in FIG. 5B.
Another inductor shown in FIG. 6A, identical to the inductor of FIG. 5A except that a meandering planar coil 9 replaced the spiral one, had an internal magnetic flux. The flux consisted of an in-plane component Bi and a vertical component Bg with respect to the soft-magnetic layers 8. These components Bi and Bg were distributed as illustrated in FIG. 6B.
From the in-plane component Bi of the magnetic flux which extending through the soft-magnetic layers 8 there was generated an eddy currents jm,p, which flowed in the direction of thickness of either soft-magnetic layers 8 as illustrated in FIG. 7. Similarly, from the vertical component Bg of the magnetic flux there was generated an eddy currents jm,i, which flowed in the surface direction of either soft-magnetic layers 8 as shown in FIG. 8.
In each of the inductors shown in FIGS. 5A and 6A, the vertical component Bg extending through the kth conductor 10 of the planar coil (6 or 9) generated an eddy current jc,l which flows along the coil conductor line 10 as shown in FIG. 9. In the spiral planar coil 6 of the inductor shown in FIG. 5A, the vertical component Bg extended in the same direction over the entire width of the coil conductor 10. Hence, as shown in FIG. 10, the density of a high-frequency current flowing through the coil conductor 10 was high at one end of the coil conductor 10 and low at the other end thereof. That is, the current density was markedly not uniform in the coil conductor 10.
In other words, the high-frequency current did not flow uniformly through the coil conductor 10. Rather, it flowed concentratedly through one end of the coil conductor 10. The resistance of the coil conductor 10 inevitably increased very much, making a large high frequency loss. This loss is considered to make it difficult to increase the quality factor Q of the planar inductor.
Furthermore, the inventors studied the increase in the high-frequency resistance of the planar coil, which had been caused by the vertical component Bg of the magnetic flux. As seen from FIG. 9, the vertical component Bg extended upwards through the kth coil conductor 10. It extended in the same direction through the same coil conductor 10. (In FIG. 9, Bgk(x) represents the density of the vertical component extending through the kth coil conductor 10.) The current flowing in the coil conductor 10 was distributed in the coil conductor 10 as indicated in FIG. 10. Namely, the current density was high in the left end of the coil conductor 10 and low in the right end thereof. This is because the eddy current jc,l generated from a vertical alternating magnetic flux was superposed on a current I supplied from an external power supply. Assuming that the density Bgk(x) of the vertical component extending through the kth coil conductor 10 is a constant one Bgk, the resistance Rc(f) the coil conductor 10 has at frequency f is given as: ##EQU1## where Rc(0) is the direct-current resistance of the coil conductor 10, tc is the thickness thereof, d is the width thereof, .rho. is the resistivity thereof, and lk is the length thereof.
The resistance Rc(f) of the coil conductor 10, calculated by the equation (1), increases with the frequency f, along a curve a shown in FIG. 11. As the curve a shows, the calculated resistance Rc(f) increases with the frequency, almost in the same manner as the measured equivalent series resistance R of the conventional planar inductor (FIG. 2), as is shown in FIG. 2 and as is indicated by a curve b in FIG. 11.
As FIG. 11 shows, the region between the calculated value a and measured value b indicates the increase of resistance R which has resulted from the high-frequency loss made at the soft-magnetic layers 8. This increase is far less than the increase in the resistance of the planar coil itself. That is, in a planar magnetic device comprising two soft-magnetic layers and a planar coil interposed between these layers, a greater part of the high-frequency loss is the loss in the coil conductor. The high-frequency loss in the coil conductor can be said to make it difficult to increase the quality factor Q of the planar magnetic device.
The conventional planar magnetic devices descried above are planar inductors. The planar transformers hitherto known have the same problem as the planar inductors. In a conventional planar transformer, the resistance of the coil conductor increases in a high-frequency band, resulting in a high-frequency loss. This loss decreases the operating efficiency of the planar transformer.